Lamps capable of producing multiple colors of light are known to satisfy many applications; including lamps for general purpose lighting that allow “white” light to be generated in such a way to allow the user to adjust the correlated color temperature of the light. Lights with adjustable color temperature are further known for use in specialized lighting applications, such as camera strobes and motion picture lighting systems. Within this application space, it is most desirable to create lamps that provide an output having both calorimetric coordinates and spectral power distributions that match those of typical blackbody radiators, typical daylight lighting, or standard daylight sources. The calorimetric coordinates of natural light that exists during the day typically fall near a curve within CIE (Commission Internationale de l'Eclairage) chromaticity space referred to as the Planckian Locus or black body curve. Methods for calculating daylight spectra for color temperatures between 4000K and 25000K, have been specified within the art, CIE publication No. 15, Colorimetry (Official Recommendations of the International Commission on Illumination), Vienna, Austria, 2004.). Standardized lighting conditions that are desirable to attain and fall near this curve; include those designated D50, D65, and D93, which correspond to daylight color temperatures of 5000K, 6500K, and 9300K, as well as so-called warmer lights, having lower correlated color temperatures, which are more similar in appearance to the light produced by tungsten lamps. In addition to having a lamp that is able to create light having the same calorimetric coordinates as these standardized lighting conditions, it is desirable to have a lamp that produces light having a spectral power distribution that matches the standardized spectral power distributions of these standardized light sources. One metric of the degree of match between the spectral power distribution of the light produced by a lamp and the spectral power distribution of these standard lighting conditions is the CIE color rendering index or CRI (CIE publication No. 13.3, Method of Measuring and Specifying Color-Rendering of Light Sources, Vienna, Austria, 1995, hereafter CIE Pub. 13.3).
The CRI is a standard method of specifying the degree to which the color appearance of a set of standard reflective objects illuminated by a given lamp matches the appearance of those same objects illuminated by light having the spectral power distribution to a specified reference source. CIE Pub. 13 provides for the computation of two color rendering indices, the so-called Special Color Rendering Index, Ri and the General Color Rendering Index, Ra. The Ri value is computed from the color difference between an individual color sample illuminated by the reference source and the lamp under test. Hence each color sample has its own associated Ri. CIE Publication 13 recommends a set of 14 color samples for testing illuminants. The General Color Rendering Index or Ra value is the arithmetic mean of the Ri values from the first 8 of these 14 color samples, and is the number commonly reported as the CRI value. Therefore, in the art the term CRI is widely used interchangeably with Ra, and will also be used in this disclosure to refer to Ra, unless otherwise noted.
In the prior art, lamps having a CRI of 80 or better provide a good match to the target spectral power distribution and are deemed to be of high quality. The disadvantages of describing the rendering capability of a light source using the CRI metric are: (1) the metric applies only to a group of eight color patches, which represent the reflectance spectrum of a limited set of reflective objects, none of which are particularly sensitive to metamerism; and (2) the metric is an average over eight Ri values, and provides only a measure of central tendency, not of consistency. As a consequence, when one applies this metric to determine the goodness of fit between a pair of spectral power distributions, it is possible, especially when using light sources whose spectral power distributions are composed mainly of a few narrow peaks, to obtain a high CRI value even though they provide a very poor match to the overall curve shape of the target spectral power distribution. For example, FIG. 10 shows two prior art spectral power distributions, a broad spectrum 154, and another spectrum 152 that consists of a series of three intense narrowband components. Both spectra have the same integrated radiant power, though clearly their curve shapes do not match well. Another consequence is that colors outside the CIE-recommended set may exhibit significant metamerism problems when using spectral power distributions composed of intense narrowband components, an effect that might not be picked up by the CRI metric. Therefore, it is important when assessing such light sources, which may be found in inorganic electro-luminescent devices, to use alternate metrics to assess the light source. Other useful metrics may include simply the root mean squared error (RMSE) between the spectral power distributions of the light being designed and the spectral power distribution of a given, typically standard lamp, when these two spectral power distributions have been normalized to have a common area under their curves. Additionally, an expanded color rendering metric may be used if additional color patches, representing the reflectance spectrum of objects that are more sensitive to metamerism are included in its calculation. Such a metric can be based on the Ri values of the additional color patches.
Lighting devices employing inorganic electro-luminescent devices have been discussed in the art that include only a few crystalline, inorganic, electro-luminescent diodes to form lamps having a high CRI. For instance, Doughty et al., in U.S. Pat. No. 5,851,063 entitled “Light-emitting diode white light source” has described light sources employing three crystalline light-emitting diodes to obtain CRI values of between 83 and 87, as well as lighting devices employing four crystalline light-emitting diodes that achieve a CRI of 96. While these devices achieve a CRI greater than 80, they do not provide a spectral power distribution that resembles the aim spectral power distribution of a black body radiator having the same correlated color temperature. This is shown in FIG. 10, which illustrates the prior art spectral power distribution 152 of a lamp having the peak wavelengths specified by Doughty and approximately the same spectral bandwidths as compared to the prior art spectral power distribution 154 of a 2800K blackbody radiator, where again, each source provides the same radiative power within the wavelength range shown. Again, the shapes of these two spectral power distributions are distinctly different from one another. As a result, when one calculates the Ri values for additional patches having spectral reflectance functions, such as those from the prior art shown in FIG. 11 as 160, 162, and 164, values as low as −60 may be obtained. A large, negative CRI typically results for illuminants containing strong narrowband components combined with spectrally selective reflectors, i.e. when light sources that have gaps in their spectra illuminate highly saturated colors. These spectral reflectance functions are from saturated cyan and purple colors that are not unusual in clothing, graphic arts or decorative materials. Further, the RMSE between the normalized spectral power distributions of the lamp provided by Doughty and the reference source is 4.9×10−3, which is relatively high. It should be noted that Doughty also provides a lamp specification including four LEDs having four unique peak wavelengths, which performs better in this respect. However, the Special Color Rendering Index for the three patches whose spectra are represented in FIG. 11 is still only 46 and the RMSE is 4.5×10−3.
A similar disclosure is provided in “Optimization of white polychromatic semiconductor lamps” by Zukauskas et al (Applied Physics Letters Vol. 80, No. 2, p. 234, 2002). This article also discusses the fact that relatively high CRI values can be obtained with light output from 2, 3, or 5 crystalline LEDs, with higher CRI values obtained for the lights with larger numbers of crystalline LEDs. However, once again the resulting spectral power distributions have a small number of narrowband components, and while they provide high CRI values when calculated using the standard eight color patches, they do not provide particularly good matches to the typically desired, daylight, blackbody, or incandescent spectral power distributions. For the light source having five narrowband components, which provides the highest CRI, the Ri values for the three spectral distributions shown in FIG. 11 are as low as 49 and once again the RMSE is large, having a value of 2.4×10−3.
It is important to note that each of these papers discuss the use of light emission from crystalline electro-luminescent diodes. These devices are typically packaged such that a single crystal, emitting light having a narrow wavelength band, is packaged as a compete device. Several of these devices are then selected and packaged together to form lamps as discussed by Zukauskas and Doughty. The requirements to form several separate packaged single crystal electro-luminescent diodes and to further select a number of these devices and package them into lamps is expensive and manually intensive, making it extremely expensive to include more than a few single crystal, electro-luminescent devices in each lamp.
Electro-luminescent devices having broader band light emission are also known. For example, Okumura in US Publication No. 2004/264193, entitled “Color Temperature-Regulable (sic) LED Light” discusses a white LED, which is formed from a phosphorescent substance that emits broadband light when excited by a blue or ultraviolet crystalline electro-luminescent device. While such embodiments provide a much broader bandwidth emission, it is not possible to tune the relative amplitude of the spectral power distribution at important locations and, therefore, it is not possible to accurately tune the output of such a device to obtain a good spectral match to a desired spectral power distribution. Further, energy is lost during the conversion of the blue or ultraviolet light to longer wavelength colors of light.
A recent article, “From visible to white light emission by GaN quantum dots on Si(111) substrate” by B. Damilano et al. (Applied Physics Letters Vol. 75, p. 962, 1999), has discussed stacking multiple layers of quantum dots, the individual layers being tuned to complementary wavelength bands, to achieve the emission of white light through photoluminescence. Electro-luminescent white light emission was not demonstrated, nor was continuous color tuning with a fixed material set. Further, this device did not match or attempt to match any desired spectrum, and the emission spectra of the devices were generally composed of a few narrowband peaks over the wavelength range that was shown.
US 2006/0043361 discloses a white light-emitting organic-inorganic hybrid electro-luminescence device. The device comprises a hole-injecting electrode, a hole-transport layer, a semiconductor nanocrystal layer, an electron transport layer and an electron-injecting electrode, wherein the semiconductor nanocrystal layer is composed of at least one kind of semiconductor nanocrystals, and at least one of the aforementioned layers emits light to achieve white light emission. The semiconductor nanocrystal layer of this device may also be composed of at least two kinds of nanocrystals having at least one difference in size, composition, structure or shape. Organic materials are employed for the transport layers, whereas inorganic materials are employed for the nanocrystals and the electrodes. While such a device may be used to create white light, it does not address the need to vary the color of this white light source or to control the spectral power distribution of the white light source.
U.S. Pat. No. 7,122,842 discloses a light emitting device that produces white light, wherein a series of rare-earth doped group IV semiconductor nanocrystals are either combined in a single layer or are stacked in individual RGB layers to produce white light. In one example, at least one layer of Group TI or Group VI nanocrystals receives light emitted by the Group IV rare-earth doped nanocrystals acting as a pump source, the Group TI or Group VI nanocrystals then fluorescing at a variety of wavelengths. This disclosure also does not demonstrate color tuning or a method to control the spectral power distribution of the white light source.
US 2005/0194608 discloses a device having a broad spectral power distribution Al(1-x-y)InyGaxN white light emitting device which includes at least one blue-complementary light quantum dot emitting layer having a broad spectral power distribution and at least one blue light emitting layer. The blue-complementary quantum dot layer includes plural quantum dots, the dimensions and indium content of which are manipulated to result in an uneven distribution so as to increase the FWHM of the emission of the layer. The blue light-emitting layer is disposed between two conductive cladding layers to form a packaged LED. Various examples are described in which the blue-complementary emission is achieved by means of up to nine emitting layers to provide a broad spectral distribution, and the blue emission is achieved by up to four blue emitting layers. However, all examples demonstrate the presence of two distinct narrowband components, to provide complementary blue and yellow colors. The authors do not discuss a means to achieve a relatively continuous broadband spectral power distribution as is required for the spectral power distributions of standard daylight, blackbody, or tungsten emitters.
There is a need, therefore, for a less expensive lamp that provides a good spectral match, especially for typical daylight or near blackbody radiators, including tungsten. As such, the lamp should provide a spectral power distribution having higher special color rendering index values and lower root mean squared errors when compared to standard near-blackbody spectral power distributions.